1. Field of the Invention
The present invention relates to pulse oximetry and, more particularly, to processing signals from a sensor.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such physiological characteristics. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.
One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry may be used to measure various blood flow characteristics, such as the blood oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time-varying amount of arterial blood in the tissue during each cardiac cycle.
Pulse oximeters typically utilize a non-invasive sensor that transmits light through a patient's tissue and that photoelectrically detects the absorption and/or scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed or scattered by the blood in an amount correlative to the amount of the blood constituent present in the blood. The amount of light absorbed and/or scattered may then be used to estimate the amount of blood constituent in the tissue using various algorithms.
Sensors exist that are designed to be applied to foreheads, digits, or various other locations on a patient's body. A phenomenon called “venous pulsation” may typically occur at sensor sites where the vasculature lacks venous valves, such as the head, but venous pulsation may also occur at less typical sites as well. Venous pulsation refers to a pulse generated from the return flow of venous blood to the heart. Because the hemoglobin in venous blood has already delivered oxygen to tissue, sensor readings based on venous pulsation may result in artificially low calculations of blood oxygen saturation (denoted as SpO2 when calculated from a pulsatile measurement). In addition, due to prominent harmonics in a venous pressure wave, pulse rate calculations based on incorrect sensor readings may be double or triple the patient's actual pulse rate. Unlike motion artifacts that may be intermittent, occurring only when a patient moves, venous pulsation can continue uninterrupted for hours. Thus, whereas inaccurate readings due to patient movement or other intermittent noise may disappear relatively quickly, inaccurate reading due to venous pulsation may last for a long time.
In some circumstances, venous pulsation may also occur at the extremities, such as fingers and feet. For example, when a patient is in an odd position, such as when a patient's extremities are placed lower than his heart for an extended period of time, venous pulsation may develop. In addition, patients with respiratory problems or patients undergoing surgery may exhibit venous pulsation even in the extremities. For example, during surgery, a patient may experience venous pulsation at a digit due to partial occlusions from a blood pressure cuff.